Motor drive devices including a rectifying circuit for converting an AC voltage from an AC power source into a DC voltage and an inverter for converting a DC voltage into an AC voltage have been widely used in a freezing devices such as air conditioners and industrial devices.
When a single phase or three-phase AC voltages are converted into a DC voltage with a diode rectifying circuit, a voltage ripple may be generated having a frequency component which is twice or six times the frequency (fs) of the AC voltage inputted into the rectifying circuit in the DC voltage. The voltage ripple can be reduced by increase in capacitance of a smoothing capacitor connected to an output side of the rectifying circuit. However, there is a problem in increase in the cost and the volume.
When the DC voltage includes the voltage ripples, this causes the output voltage of the inverter to include a “difference frequency component” between a DC voltage ripple frequency (fr) component and the output frequency component of the inverter (|fr−fl|) due to detection delay of the DC voltage and operation delay in an inverter controller in addition to the output frequency (fl) component of the inverter.
When the output frequency of the inverter approaches the DC voltage ripple frequency, a beat phenomenon is caused by “difference frequency component” when a resistance or inductances of windings of the motor driven by the inverter are low, which generates a large pulsate current, so that an output torque of the motor pulsates. Particularly, at an over-modulation region of the inverter, the beat phenomenon becomes remarkable because there is a section in which amplitude of the output voltage cannot be adjusted.
For example, Patent document 1 (JP2004-104898 A) disclosed as a method of suppressing a current beat phenomenon a method of correcting a three-phase voltage command value as a method of suppressing the beat phenomenon by detecting a high frequency component of a γ-δ axis current on a rotary coordinate system, calculating a three-phase current beat components, and amplifying the operated value.
Patent document 2 (JP2008-167568 A) disclosed a method of suppressing the current beat by correcting an output voltage of the inverter through a process of multiplying, etc. a motor current detection value by a sine (sin) signal and a cosine (cos) signal.
Further, Patent document 3 (JP2015-42010 A) disclosed a method of correcting each of phase voltage commands by detecting pulsating component of a low frequency through a periodic integration process of each of phase output voltages of the inverter.